Amplifier volume control



May 19, 1936. H. A. WHEELER AMPLIFIER VOLUME CONTROL 2 Shevets-Sheet 1 Original Filed July '7, 1927 Nx NESS@ s AAM vvvv v ATTORNEYS May 19, 1936. H. A. WHEELER AMPLIFIER VOLUME CONTROL 2 sheets-sheet 2 y Original Filed July 7, 1927 ATTORNEYS Patented May 19, 1936 UNITED STATES PATENT oFFlcE 2,041,213 y AMPLIFIER `VOLUliIE CONTROL .Harold A. Wheeler, Great Neck, N. Y., assigner to Hazeltine Corporation, a corporationv ofl Delaware 12 Claims. (Cl. Z50- 20) This invention relates to amplifiers, and more particularly to amplifiers utilized in modulated carrier-current signaling systems wherein the limit of amplification is automatically maintained substantially at a predetermined level.

'I'l'iisapplication is a division of application Serial No. 203,879, filed July '7th, 1927.

When amplifiers are employed for amplifying a. signal voltage it becomes desirable for various lo reasons to control automatically the 'amplitude of this amplified signal voltage. To this end the present invention provides means for effecting automatic amplification control. Such an arrangement, for example, is particularly advantageous in radio receivers such as are employed for receivingrbroadcast signals, because. it prevents the overloading of the last amplifier stage of the receiver, which overloading would result in distortion of the reproduced signal, as well as loud and harsh reproduction.

Another advantage resides in uniform reproduction of the amplified signal irrespective of whether the carrier-current signal is received from a nearby station or from a distant station, or from a. high-power station, or a low-power station, since it has been found in former radio .receivers that when the receiver was reproducing strong signals as `from a nearby, or a high-power station, the audibly reproduced signal was very loud, whereas when the signal was received from a distant, or a low-power station, it was relatively weak, with the result that if signals, were to be reproduced .uniformly from both near and distant stations, and from high-power and low-power stations, it became necessary to readjust some volume controlling means in the receiver to compensate for these unequal signals.

It has also been a common experience in the use of former radio receivers that the reproduced signal was not uniform due to the phenomenon of fading, whereby the received signal occasionally, orperiodically, became much weaker, or faded almost to the point of inaudibility. Since the present invention provides an amplifier which automatically compensates for inequalities in the received carrier-current signal strength, when fading takes place the degree of amplification is correspondingly increased and the reproduced signal maintained at its former volume, so that a listener is unaware that variation of the received carrier-current signal is occurring. This automatic compensation for signal fading is espe-v cially advantageous in commercial radio telephony and like systems. I i

In present-day radio receivers in which operating current lis derived from the municipal power system, it has been found that when there is considerable variation in the. line voltage supply, the volume of the reproduced signal isnot uniform. An additional advantage of the present invention is that of automatically compensating for such line voltage variations with the result that the reproduced signal'is uniform in volume.

A still further advantage is the saving in plate current which is automatically effected during the reception of powerful signals, for the reason that this invention incidentally provides means for reducing the plate current of one or more amplifying tubes as the signal strength increases.

Fig. 1 is a circuit diagram of a complete radio receiver which includes the present invention, and consists of a three-stage radio-frequency amplifier followed by` a rectier, a two-stage audio-frequency amplier, and a loud speaker, or other suitable indicating device.

Fig. 2 shows curves disclosing the relation between the radio-frequency antenna voltage and the radio-frequency amplified voltage, with and without the application of the present invention.

Fig. 3 shows a circuit diagram of a second embodiment of the invention in which there is disclosed a three-stage tuned radio-frequency amplifier, a rectifier, and a three-stage audio-frequency amplifier.

Fig. 4 shows graphically a comparison between the performance of the two-electrode valver or rectifier, and of the three-electrode detector.

Referring in detail to Fig. 1, there is shown an antenna t connected to ground I through the primary winding 6 of a radio-frequency transformer, the secondary winding 1 of which, tuned by a variable condenser 8, is connected at one point to the lament of the vacuum tube 9 in the first radio-frequency amplifying stage and at another point to the grid II of this vacuum tube. The output circuit of this vacuum tube extends from the filament system, through a high-voltage battery B, a milliammeter I0, primary winding I3 of a second radio-frequency transformer to the anode or plate I4 of this vacuum tube. In order to neutralize the inherent capacity between the grid I I and the plate I4, and thereby to prevent oscillations, and otherwise to increase the effectiveness of the present invention as hereinafter described, a neutralizing winding I9, electromagnetically coupled to winding I3, and a neutralizing condenser 3 are employed in the manner described in the U. S. patents to Hazeltine Nos. 1,489,228 and 1,533,858.

A second stage of radio-frequency amplification including the vacuum tube I 5 neutralized by cooperation of cbil and condenser 4, like the first stage, comprises the 'secondary winding I6 of the last-mentioned radio-frequency transformer tuned by a variable condenser I1 connected between the filament system of the vacuum tube I5 and the grid I8 thereof. The output circuit of this vacuum tube also includes the high-voltage battery B and a primary winding 20 of a second radio-frequency transformer, while the secondary winding 2I of this transformer tuned by a variable condenser 22 is included in the input circuit of, a third stage of radio-frequency amplification which includes vacuum tube 23. The inherent capacity effective between the electrodes 24 and 25 is neutralized by a network including the neutralizing condenser 2B and the neutralizing winding 29 as described in the mentioned patents.- The output circuit of the vacuum tube 23 includes the primary winding 30 of a third radio-frequency transformer and the high-voltage battery B. The secondary winding 3I of this last-mentioned transformer, tuned by a variable condenser 32, is connected in the input circuit of a rectifier 33 which input circuit includes the iixed condenser 2. The rectifier employed may be of the type commonly known in the art as a two-electrode "Fleming valve, orv may consist of an equivalent such as a three-electrode vacuuml tube, as shown, having its grid I2 and its plate or anode 35 directly connected together to comprise in effect a single anode.

In the absence of the present invention including the control circuit 36, to be described, the three-stage tuned radio-frequency amplifier, including the vacuum tubes 9, I5, and 23, functions in a manner well-known in the art to amplify selectively the incoming modulated-carrier signals of any carrier amplitude within a wide range, which are intercepted on the antenna 5. The output circuit of the rectifier 33 includes what may be termed a rejector circuit for stopping radiofrequency currents which have passed through the rectifier, and consists of a network including a resistance 34 and a by-pass condenser 31 connected between the anode 35 and the filament 38 of the rectifier. The output circuit of the rectifier is coupled to the input,circuit of an audiofrequency amplifying-vacuum tube -39 through an audio-frequency-pass lter including a fixed condenser 40 and an impedance 4I connected between the filament 42 and the grid 43 of this vacuum tube. As appears vfrom the constants hereinafter given, the characteristics of this lter are such that it passes the audio-frequency component to the input circuit of the audioor modulation-frequency amplifier, while preventingv the unidirectional component from being impressed upon the input circuit thereof. The output circuit of this amplifier is connected between the filament 42 and plate 44 through the high-voltage battery c B and the primary winding 45 of an audio-freare given herewith.

output circuit of thelast audio-frequency amplitying tube 41. It is presumed that adequate precautions against undesired electromagnetic coupling between the various radio-frequency coupling transformers are included in all of the arrangements herein disclosed. A

In accordance with the main feature of the present invention, means are provided to control automatically the degree of amplification eiected in the radio-frequency amplifying stages. These means include a resistance 5I, connected between the filament 33 and the anode, or output electrode, 35 of the recter, through which the pulsating rectified or 4converted current flows, thereby developing a negative voltage at terminal 52. This negative voltage is applied over conductor 36 through the resistance 53 and the secondary winding 1 of the first radio-frequency transformer to grid II of the first radio-frequency stage. Resistance 53, together with blocking condenser 54, is effective in producing a time constant predetermined to filter out and reject voltage fluctuations at frequencies of modulation of the selected and amplified signal, that is, any audio-frequency currents winch otherwise might be applied from conductor 36 to the grid II.

To complete the description of the system illustrated in Fig. 1 certain design data or constants -It should be understood, however, that these, as well as all other constants appearing in the present specification, are mentioned merely by way of example in describing certain specific embodiments which in practice have proved eminently satisfactory, and are not intended to suggest any specific limitations as to the scope of this invention. Accordingly, fixed condenser 2 may be of 0.0005 microfarad; 31 of 0.0001 microfarad; 54 of 0.01 microfarad; 4I) of 0.005 microfarad; resistance 5I of 1 megohm; 34 of y1 megohm; and 4I and 53 of 2 megohms each.

In the operation of the receiver shown in Fig. 1, a signal modulated-carrier intercepted on the antenna 5 is selected and successively amplified through the neutralized radio-frequency stages indicated by the vacuum tubes 9, I5, and 23 connected in cascade. This amplified signal voltage is then rectified by the rectifier 33, and the rectified pulsating current is successively amplified by the audio amplifying stages including vacuum tubes 39 and 41, after which it may be reproduced as sound by the loud speaker 50.

'Ihe high resistance 5|uconnected to the cathode or filament 38 and the anode 35 of the rectifier, through which a small space current flows in absence of signal output from the radiofrequency amplifier, maintains the anode normally negative relative to at least a part of the filament of the rectifier. Since all the filaments are connected in parallel, the rectifier filament is maintained at substantially the same potential as the filament 21 of the first radio-frequency amplifier tube 9. Therefore, the resistance 5I is connected effectively between the rectier anode 35 and the amplifier filament 21, and thereby determines the normal potential of the rectifier anode negative relative to at least a part of the amplifier filament or cathode. 'I'he resistance 5I being on the order of 1 megohm, which is of a higher order than that of the resistance to current in the more conductive direction of ordinary commercial types of vacuum tubes used as diodes, is therefore a controlling factor in affecting the operation of the rectifier circuit. By virtue of the proportions of the condenser 2 and the resistance i for example as given above, and the resistance of the rectifier tube 35, the rectifier circuit has a time constant substantially less than the period of the highest -frequency of strong modulation.

When the rectified or converted signal voltage across the resistance 5| increases with signal output beyond a predetermined value, there is developed at the anode terminal 52 sufficient'negative biasing voltage which inturn is impressed, through the conductor 36, upon the grid, or control electrode, I I `of the vacuum tube 9, to reduce the amplification of this tube. Conversely, it-will be apparent that as the magnitude of the rectilied current flowing through resistance 5| decreases with decreasing signal strength, the direct voltage at terminal 52 becomes less negative, and the negative biasing voltage impressed upon the grid Il also diminishes so that the vacuum tube 9 effects an increased degree of amplification. In this manner, theradio-frequency voltage applied to the input of the rectifier is maintained at a nearly constant predetermined value. and the volume of the reproduced signal is substantially uniform under allconditions. In other words, the carrier amplitude at the rectifier is maintained above a predetermined lower limit, or Within a range ,having la predetermined lower limit, the rectifier circuit by virtue of the aforementioned proportions of its component elements having the characteristic ofv linear rectification of a strongly modulated signal of any carrier amplitude within this range. The level at which the volume is maintained uniform is determined by adjustment of rheostat 4,9 which controls the heating current in the filament 42 of the first audio-frequency amplifying tube 39.

In the above operation it is noted that the twoelectrode or diode rectifier 33 functions as the detector and also effects rectification of the carrier current to control the amplification in the carriercurrent amplifier of the receiver. The audiofrequency component ofthe detector output is coupled to the input circuit of the audio-frequency amplifier for further amplification.

The neutralization of the grid-plate capacity of the radio-frequency amplifying tubes is, in

combination with the present invention, particui larlyvaluable in that it allows an increase in the effectiveness of the amplification control, because such neutralization prevents radio-frequency energy from passing through the grid-plate capacity of the tubes. Thus the relayaction of the tubes is almost entirely subject to the control by grid bias voltage provided in accordance with this invention.

The time required for operation of the control system would ordinarily be determined by the lowest audio-frequency modulationwhich must be reproduced. Fading, for example, might be considered a form of ,modulation; the frequency of the rise and fall of -signals due to fading being the frequency of modulation. If this frequency of modulation 'be increased sufiiciently,I the effect -will be audio-frequency modulation. It will thus is chosen which will be greater than the period of the audio frequencies which the system is intended to amplify. 'Ihis time constant of the control circuit is equal to the product of the series reistance and the shunt capacitance of milliammeter lll so indicates.

the grid bias circuit, represented in Fig. 1 by resistance 53 between the conduiltor 36 and the anode terminal 52 in the direct-current connection back to the grid ii,. and condenser 54 connected between the amplifier filament 21 and a reproduction of music, there appears to` be no need for more rapid control under the conditions usually encountered. The use in this connection of condensers of large capacitance, such as onetenth microfarad. likewise introduces another convenience in that the condensers 'may also serve to by-pass radio frequencies in order to prevent undesired coupling between the detector circuit and the rst radio-frequency amplifying tube because of some impedances common to those two portions of the apparatus. y

The milliammeter I0 is connected in the anode circuit of the amplifying vacuum tube 9. On receipt of an amplified signal at the detector, the effect of the control circuit is to reduce the amplification in the tube 9, and incidentally to reduce the plate current through the milliammeter I0. When the receiver is tuned to the signal frequency, a minimum amplification in this tube is required, so that the plate current of tube 9 then reaches a minimum value, and the Thus the milliammeter visually indicates the condition of resonance.

For a better understanding ofthe present invention reference is made to Fig. 2 from which it will be appreciated that in a system similar to that illustrated in Fig. 1, but in which no means for automatically limiting the degree of amplification is included, theamplfiedradi-frequency voltage is proportional to the radio-frequency antenna voltage, as indicated by curve |62.

When, however, the present invention is em- V ployed in such an amplifier, the relation between the radio-frequency antenna voltage and the amplified radio-frequency voltage is indicated by curve |03 from which it will be seen that `when at least a. certain predetermined radiofrequency antenna voltage is present, (herein referred to as the threshold antenna voltage) the amplified radio-frequency voltage appreaches-but is always less thananother certain predetermined voltage value (herein` referred to asthe cut-off voltage).`

The modification illustrated in Fig. 3 is an especially desirable form of the present invention, and includes antenna 56, connected by means of a transformer 51 to a neutralized three-stage tuned radio-frequency cascade amplier including the vacuum tubes 58, 6D, and 62 coupled by transformers 59 and 6I. The last stage of the amplifier is connected by a transformer 63 to a two-electrode rectifier 64 of the F type already described, the output circuit of which, including the resistance 65, is connected between the anode 66 and filament 61 of theV rectifier, as previously explained. Resistance 'l2 and condenser 68 associated with this output circuit, constitute a "rejector" network which filters out the radio-frequency current component in the output circuit of the rectifier 8l, while the network including condenser 68 and resistance 18 constitutes an audio-frequencypass filter for coupling the output circuit of the rectifier to the inputcircuit of the audio-frequency amplifier which includes vacuum tube 1|. Rheostat 13 controls the heating current supplied to the, filament 14 of this vacuum tube, and thereby permits a manual adjustment of the volumeof the reproduced signal desired by the listener. Audio-frequency transformer 18,

. which is preferably of a low ratio of transformal ing the vacuum tubes 58, 68, and 62, of which two are controlled in accordance with the present invention. yThe amplified radio-frequency current is rectified by the rectifying valves, and successively amplified at audio-frequency by the vacuum tubes 1|, 11, and 18. 'I'he rectified current in the output circuit of the rectifier fiows through the resistance 65, and thereby develops a negative voltage at the terminal 8|, which voltage is applied through the impedances 12, .82,V 83, and 85 to the grids 84 and 86 of the radio-frequency amplifying tubes 58 and 68. By thus simultaneously controlling the degree of amplification of two successive radio-frequency amplifying stages va greatly increased uniformity of regulation is attained. Resistance 82 and the condenser 81 constitute an audio-frequency-stop filter, so that substantially only direct-voltage is impressed upon the grids 84 and 86. It will be understood that the voltage developed at terminal 8| is afunction of the amplified radio-frequency voltage delivered to theinput circuit of the rectifier by the radiofrequency amplifying tubes 58, 68, and 62, and

therefore, as the negative voltage at terminal 8| tends to increase with the increased signal, the resulting increase of biasing voltage impressed upon the grids of the tubes 58 and 68 limits the degree of amplification effected in the radio-frequency stages including those tubes.

In this arrangement the constants for the various resistances and condensers may, for example, be the same as those for the corresponding elements in Fig. 1. In addition the grid resistances 83 and 85 may have a value of 2 megohms each; and the grid condensers connected at the junction of these resistances and the grid electrodes 84 and 86 may each be of 0.001 microfarad capacity.

In Figs. 1 and 3 the variable tuning condensers are grounded in order to eliminate undesired capacity effects as Well as to make it practicable to connect the condensers on a. single shaft for unicontrol, if desired.

'I'here are advantages attending the use, in connection with the present invention, of the twoelectrode rectifier circuit typified b y Figs. 1 and 3, which may not be apparent from the foregoing discussion. It is impossible to overload this type of rectifier, and the rectied output voltage is directly proportional to the applied alternating signal voltage when this voltage is large, say over two volts. ,Ihe control system in the circuits of the figures referred to requires a large operating voltage, say ten volts, so that the latter condition of large signal voltage is realized. No such simple relationship is possible in a three-electrode detector, whose rectified output never exceeds a. limiting upper value, and is never proportional to the applied voltage, except over a very small range of voltages. This distinction will be seen from Fig.,4 where the abscissae A. C. represent the alternating signal voltages, whereas the ordinates D. C. represent the rectified output voltages. It is well known that the linear curve is much more desirable when minimum distortion of a modulated signal is desired, and it will be observed from Fig. 4 that the preferred type of curve is obtained from thetwo-electrode rectifier.

A further advantage of the linear type detector with the automatic volume control connection and a visual resonance indicator in the anode circuit of the amplifier whose grid bias is being automatically controlled, lies in the fact that the visual resonance indicator will give an indication which is proportionate to the received signal intensity. This follows from the fact that the negative grid bias on the amplifier is directly proportional to the strength of the signal; and hence the anode current bears a similar relation to the signal.

While a three-electrode detector is useful in certain arrangements for relatively small applied voltages, the rectified output voltage is approximately proportional to the square of the applied voltage, i. e., to the power associated with the applied voltage. For this reasonV the rectified voltage increases with the carrier wave modulation.

When such a detector is used in an automatic amplification control system, the total power from the radio-frequency amplifier is maintained at a substantially constant level, the amplitude of the carrier wave being decreased in the presence of modulation. It is desirable to maintain the carrier wave at a constant amplitude at the output of the amplifier, and this is accomplished .by the diode rectifier as shown in Figs. 1 and 3. The control system maintains constant the average signal amplitude which is equal to the carrier wave amplitude and independent of the degree of modulation.

It will be observed that in a system employing a two-'electrode rectifier such as represented by valve 88 of Fig. l, and 64 of Fig. 3, the control bias voltage is independent of the B or anode battery voltage. Since the diode rectifier is not an amplifier, is not critical, and requires neither anode nor biasing battery, no adjusting devices are required.

In the foregoing description, tuned radio-frequency receivers of the neutralized types have been referred to. It should be pointed out, however, that the present invention may be employed with equal effectiveness to any radio receivers in wired radio and space radio systems, and that it has been found especially applicable to receivers of the super-heterodyne type.

It is well known that the common B battery may be replaced by a source of rectified and filtered alternating current, and, in the event that tubes having indirectly heated cathodes are used instead of those having incandescent directly heated filament cathodes, the common A battery may be replaced by a source of alternating current.

ampare.

1. An audion `amplifier including an audionv for signal wave amplification, an output circuit, and means limiting fluctuations in said output circuit due to variations in theA strength of an incoming signal, said means comprising a diode rectifier for-incomingsignal energy, and means for impressing upon the grid of said audion a di'- rect current bias voltage derived from said rectier, said rectifier.4 being of the type having approximately linear relation between direct current output and radio frequency input.

2. In a carrier wave receiving system, a rectifier of received signals coupled toav signal indicating device, and means automatically maintaining the output of said rectier within a narrow range for wide variations in amplitude of the received carrier, the output of said rectifier being, throughout saidnarrow range, linearly related to its input, thereby to minimize distortion of signals relayed to said indicating device.

3. In a carrier wave receiving system, a rectifier of received signals coupled to a signal indicating device, the output of said rectifier being, over a limited range, linearly related to its input, and means automatically maintainingpthe output of said rectifier within said linear range` for wide variations in amplitude of the received carrier, thereby to minimize distortion of signals relayed to said indicating device.

4. In a carrier wave receiving system in cascade, a vacuum tube having a control grid, 'a rectifier, and a signal indicating device, and means i applying to said control grid a biasing voltage derived from said rectifier for automatically maintaining the rectified output within a narrow range for wide variations in amplitude of the received carrier, the output of said rectifier being, throughout said narrow range, linearly related to its input. i

5. In a carrier wave receiving system in cascade, a vacuum tube having a control grid, a rectifier, and a signal indicating device, the output of said rectifier being throughout aw limited range, linearly related to its input, and means for applying to said control grid a biasing voltage derived from said rectifier for automatically maintaining the outputof said rectifier Vwithin said linear range for wide variations in amplitude of the received carrier.

6. In a. carrier wave receiving system, a rectifier, means relaying high frequency signals thereto including a vacuum tube having a control grid, means applying to ksaid control grid a biasing voltage derived 'from said rectifier for automatically maintaining vthe rectified output within a range which is small compared to variations in carrier amplitude impressed on said system, the output of said rectifier being throughout said small range, linearly related to its input, whereby `said biasing voltage is determined solely by the carrier amplitude unaffected by modulation thereof.

:7. In a carrier wave receiving system, a rectifier, means relaying high frequency signals thereto including a vacuum tube having a control grid, the output of said rectifier being, throughout a. limited range, linearly related to its input, and means for applying to said control grid a biasing voltage derived from said rectifier for automatically maintaining the rectified output within said linear range for wide variation in amplitude of the received carrier, whereby said biasing `voltage is determined solely by theV carrier amplitude unaffected by modulation thereof.

8. In a modulated-carrier signal receiver for receiving a signal of any carrier amplitude within a wide range, a rectifier having the characteristic of linear rectification of a strongly modulated signal of any carrier amplitude above a predetermined lower limit, and means responsive to the carrier amplitude for automatically maintaining the carrier amplitude at said rectifier within a range determined by said lower limit and the overload point of the receiver.

9. In a modulated-carrier signal receiver for receiving a signal of any carrier amplitude within a wide range, a diode rectifier having the characteristic of linear rectification of a strongly modulated signal of any carrier amplitude above a predetermined lower limit, the circuit of said rectifier having a load impedance much greater than the diode resistance to current in the more conductive direction, and means responsive to the carrier amplitude for automatically maintaining the carrier amplitude at said rectifier Within a range immediately above said lower limit and of lower order of magnitude than the range of the received signals;

10. In a modulated-carriersignal receiver for receiving a signal of any carrier amplitude within a wide range, an amplifier in which the amplification varies with a bias voltage applied thereto, a rectifier having the characteristic of linear rectification of a strongly modulated signal of any carrier amplitudeabove a predetermined lower limit,` means for coupling the output of said amplifier to said rectifier, and means for applying to said amplifier as a bias voltage a rectified-carrier voltage from said rectifier,` and thereby automatically maintaining the carrier amplitude oi' said amplifier output within a range predetermined by said lower limit and the overload point of the receiver.

11. In a modulated-carrier signal ,receiver for receiving a signal of any carrier amplitude within a wide range, an amplifier in which the amplification varies with a bias voltage applied thereto, a rectifier having the characteristic of linear rectification of a strongly modulated signal of any carrier amplitude above a predetermined lower limit, said rectifier circuit having a time constant substantially less thanthe period of the -highest frequency of strong modulation, means for coupling the output of said amplifier to said rectifier, and a coupling circuit for applying to said amplifier a bias voltage which varies with the rectified carrier voltage from said rectifier, said amplifier, rectifier andl coupling circuit being relatively proportioned to maintain the carrier amplitude at said rectier within a range immediately above said lower limit and of a lower order of magnitude than the rangeof the received signals, and said coupling circuit having a time 'constant greater than the period of the lowest frequency of strong modulation, for causing said bias voltage to vary only with the signal carrier amplitude and not with signal modulation.

12. In a modulated-carrier signal receiver for receiving a signal of any carrier amplitude within a wide range, a diode rectifier, a condenser connected in series with said rectier, a leakage path across said rectifier having to modulationfrequency current and directrcurrent a substantially uniform resistance'of a higher order than that of said rectifier to current inV its more conductive direction. whereby said rectifier has the characteristic ot linear rectincation of a strongly modulated signal of any carrier amplitude above a predetermined lower limit. and means responsive to the amplitude for autnatically maintaining the input carrier amplitude at the rectiner within a range immediately above said lower limit and oi! a'lower order of magnitude thantherange oftherecelvedsignals.

' HAROLDA. 

